The present invention relates generally to x-ray systems and, more particularly, to a method and system of aligning an x-ray detector such that the x-ray detector is perpendicular to and properly spaced from an x-ray tube during data acquisition.
X-ray imaging is a non-invasive technique to capture images of medical patients for clinical diagnosis as well as inspect the contents of sealed containers, such as luggage, packages, and other parcels. To capture these images, an x-ray source irradiates a scan subject with a fan beam of x-rays. The x-rays are then attenuated as they pass through the scan subject. The degree of attenuation varies across the scan subject as a result of variances in the internal composition of the subject. The attenuated energy impinges upon an x-ray detector designed to convert the attenuating energy to a form usable in image reconstruction. A control system reads out electrical charge stored in the x-ray detector and generates a corresponding image. For a conventional, screen film detector, the image is developed on a film and displayed using a backlight.
Increasingly, flat panel, digital x-ray detectors are being used to acquire data for image reconstruction. Flat panel detectors are generally constructed as having a scintillator which is used to convert x-rays to visible light that can be detected by a photosensitive layer. The photosensitive layer includes an array of photosensitive or detector elements that each store electrical charge in proportion to the light that is individually detected. Generally, each detector element has a light sensitive region and a region comprised of electronics to control the storage and output of electrical charge. The light sensitive region is typically composed of a photoconductor, and electrons are released in the photoconductor when exposed to visible light. During this exposure, charge is collected in each detector element and is stored in a capacitive element (diode) situated in the electronics region. After exposure, the charge in each detector element is read out using logic controlled electronics.
Each detector element is conventionally controlled using a transistor-based switch. In this regard, the source of the transistor is connected to the diode, the drain of the transistor is connected to a readout line, and the gate of the transistor is connected to a scan control interface disposed on the electronics in the detector. When negative voltage is applied to the gate, the switch is driven to an OFF state, i.e. no conduction between the source and drain. On the other hand, when a positive voltage is applied to the gate, the switch is turned ON resulting in connection of the source to the drain. Each detector element of the detector array is constructed with a respective transistor and is controlled in a manner consistent with that described below.
Specifically, during exposure to x-rays, negative voltage is applied to all gate lines resulting in all the transistor switches being driven to or placed in an OFF state. As a result, any charge accumulated during exposure is stored in each detector element capacitor. During read out, positive voltage is sequentially applied to each gate line, one gate at a time. In this regard, only one detector element is read out at a time. A multiplexer may also be used to support read out of the detector elements in a raster fashion. An advantage of sequentially reading out each detector element individually is that the charge from one detector element does not pass through any other detector elements. The output of each detector element is then input to a digitizer that digitizes the acquired signals for subsequent image reconstruction on a per pixel basis. Each pixel of the reconstructed image corresponds to a single detector element of the detector array.
As described above, for indirect detection, digital x-ray detectors utilize a layer of scintillating material, such as Cesium iodide (CsI), to convert incident radiation to visible light that is detected by light sensitive regions of individual detector elements of a detector array. Generally, the transistor controlled detector elements are supported on a thin substrate of glass. The substrate, which supports the detector elements as well as the scintillator layer, is supported by a panel support. The support panel is not only designed to support the detector components, but also isolates the electronics for controlling the detector from the detector components. The electronics are supported by the base of a cover assembly enclosing the internal components of the x-ray detector.
It is generally well-known that radiation detection efficiency and resulting image quality are, in part, a function of the orientation of the x-ray detector relative to the x-ray tube during data acquisition. In a cross-table x-ray scan, it is preferred that the plane of the x-ray detector be perpendicular to the plane of the beam of x-rays emitted by the x-ray tube. If the alignment of the x-ray detector and the x-ray tube are not generally perpendicular, dose may not be uniform across the x-ray detector yielding an image prone to artifacts. As a result, multiple x-ray images may be needed to reconstruct a diagnostically valuable image, which requires additional subject exposure to radiation and negatively affects subject throughput.
There is a general need to develop a system to assist a user in properly aligning an x-ray detector and an x-ray tube relative to one another. Presently, a user typically relies upon experience when positioning an x-ray detector and x-ray tube relative to one another. While for some x-ray scans this is feasible, when the x-ray detector and x-ray tube are to be positioned at an angle, achieving a proper alignment can be particularly difficult. As a result, a user must subject the subject to an x-ray scan, determine if the image is of sufficient image quality, and, if not, re-acquire data from the subject.
Therefore, it would be desirable to design a system and method of assisting a user in positioning an x-ray tube and x-ray scanner relative to one another.